1. Field of the Invention
The present invention relates to composite membranes, methods for making composite membranes and applications of the composite membranes. The composite membranes include a porous support structure and one or more active layers disposed within the pores of the support structure.
2. Description of Related Art
Efficient and cost-effective membranes are needed in many applications, including separation and purification of gases, such as the purification of hydrogen (H2) for use in fuel cells and in point-of-use applications. As an example, it is often necessary to remove contaminant gases such as carbon monoxide (CO) from a gas stream containing H2. Some membranes include a membrane support and an active layer, where the active layer is permeable to only species that are desired to go through the membrane, commonly referred to as supported membranes. In other cases, the entire membrane body serves as a separating layer, commonly referred to as bulk membranes.
For H2 separation, membrane active layers of metals and metal alloys, particularly those including palladium (Pd), are impervious to all gas species except H2 and thereby separate the H2 from the other gases. Such membranes can be fabricated in the form of self-supporting bulk foils. Although Pd-based bulk foils exhibit near-infinite selectivity for H2, they are expensive and have poor flux due to the required foil thickness.
Active membrane layers can also be supported by porous substrates and thin Pd-based supported films can be used to increase membrane flux. However, the fabrication of thin-film Pd supported membranes that have the required defect-free structure requires a Pd thickness of at least 10 μm to 50 μm, which is too thick for many applications, such as H2 separation in portable fuel cell reformers. Furthermore, the reliability of supported membranes is limited by the poor mechanical integrity of the thin metal layers deposited onto the porous support. Further, the poor mechanical integrity is often exacerbated by temperature cycling and/or mechanical loads that are encountered in use. Also, the reliable sealing of thin supported membranes is also challenging and the cost of the manufacturing and integration of such membranes has hindered their widespread application.
Recently, MEMS technology has been applied to supported membranes to generate defect-free high permeability membranes, as is reported by Karnik et al. (“Towards a palladium micro-membrane for the water gas shift reaction: microfabrication approach and hydrogen purification results”, Journal of Microelectromechanical Systems, February 2003, Vol. 12, Issue 1, pgs. 93-100). Submicron-thick Pd “windows” produced on etched silicon wafers demonstrated large hydrogen flux as a function of Pd area and demonstrated high selectivity. However, the total area of the supported Pd membrane was small, limiting the total flux. Additionally, the Pd windows ruptured when subjected to transmembrane pressures of about 0.5 bar, and the thermal reliability of the thin Pd film on Si was a problem due to the mismatch of temperature expansion coefficients.
Although thin-film supported membranes, such those described above for H2 separation, have been fabricated, their commercial utility has not been realized. Such membranes have problems related to poor adhesion of the Pd layer to the support, damage to the Pd layer caused by thermal cycling and susceptibility to damage from mechanical abrasion
Films of Anodic aluminum oxide (AAO) includes elongate mesopores that extend through the entire thickness (Furneau et al, Nature, 71, p. 337 (1992)), and has been utilized as a substrate for different types of membranes. For example, Pd films as thin as 200 nm have been sputtered onto the surface of AAO for a H2 separation membrane, as is reported by Konno et al. (“A Composite Palladium and Porous Aluminum Oxide Membrane for Hydrogen Gas Separation”, J. Membr. Sci., Vol. 37, pp. 193-197, 1988) and Mardilovich et al. (“Gas Permeability of Anodized Alumina Membranes with a Palladium-Ruthenium Alloy Layer”, Russian J. Phys. Chem., Vol. 70, pp. 514-517, 1996). The resulting membranes exhibit high selectivity and permeability for H2. However, although these membranes could provide much thinner active layer, the active layer is still on the membrane surface and is prone to hydrogen embrittlement and mechanical damage.
Itoh et al. (“Deposition of Palladium Inside Straight Mesopores of Anodic Alumina Tube and its Hydrogen Permeability”, Micropor. and Mesopor. Mat. and Chem. Res., Vol. 39, pp. 103-111, 2000) report that Pd was deposited inside the pores of AAO for the fabrication of membranes for the separation of H2. Fabrication of these membranes involved sputtering of a conductive contact from Pd, Pt or Ag onto one of the surfaces of the blank AAO membrane, followed by the electrodeposition of Pd, resulting in an active layer comprised of the Pd deposited onto the contact film on the membrane surface as well as inside the AAO pores. The method does not allow the formation of the active layer disposed entirely within the nanoporous support structure.